1. Field of the Invention
This invention relates in general to systems and methods for determining fluid volume, detecting fluid leaks, and monitoring fluid inventory in one or more tanks by the use of ultrasonics, and more particularly, as these systems and methods are applied to underground storage tanks (UST).
2. Prior Art
The need to accurately measure the volume of a fluid in a vessel, such as an underground gasoline tank, has long been recognized. Perhaps the oldest, and, at least at one time, the most common way of determining the volume of fuel was to insert a gauge stick into the tank and read the height of the fluid from the line formed on the rod by the fuel's surface. However, this method only provides course estimates of the volume that are no longer acceptable in meeting today's environmental requirements.
In an effort to overcome the various shortcomings of the gauge stick, other devices and methods have been tried. One of these is the use of ultrasonic ranging systems. Examples of the earlier ultrasonic ranging systems can be found in U.S. Pat. Nos. 3,113,456; 3,184,969; and 3,394,589. In these systems the transducer was placed above the fuel surface and measurements were made from the top of the tank to the fuel surface. This measure was then subtracted from the known distance of the bottom of the tank to the top of tank to obtain the level of the fuel in the tank. From that calculation and the known geometry of the tank, the volume of fuel in the tank could be calculated. However, because of variance in the fuel surface level due to temperature fluctuations in the tank, variance of the speed of sound depending on the medium in which the sound waves were travelling, as well as, the accumulation of water in the bottom of the tank, these systems could not provide the accuracy now demanded.
In an attempt to overcome some of the difficulties of these earlier ultrasonic ranging systems, reference reflectors were added to provide data which could compensate for the environmental conditions within a fuel tank. In addition, the transducer began to be placed within the fuel rather than above the fuel in order to facilitate the determination of how much water had accumulated in the bottom of the tank. Examples of these systems can be found in U.S. Pat. Nos. 4,170,765; 4,210,969; 4,337,656; 4,470,299; and 4,578,997. Although these systems represented an improvement over the prior art systems of that time, the combination of tank geometry, variations in the speed of sound through the propagation medium, the electronic circuitry, and the probe design still limited the performance of such systems.
In 1988 the U.S. Environmental Protection Agency (EPA) finalized regulations for underground storage tanks (UST) containing petroleum or other hazardous substances. The regulations established the requirement that release detection procedures for USTs must be implemented. In response to these regulations six general categories of release detection methods have been developed: tightness or precision testing, tank gauging systems, inventory control methods, ground-water monitoring, vapor monitoring, and interstitial monitoring.
The last three categories are secondary methods; that is, the product release is detected via external sensors placed in the immediate area of the tanks and the piping system. This invention can work in conjunction with these secondary methods. On the other hand, the first three categories for leak detection are primary - methods; that is, these methods provide direct evidence of the loss of a product.
For inventory control purposes, these EPA regulations require that the system must make a gasoline level measurement and interval temperature measurements so as to compensate the inventory to a reference temperature (typically 68.degree. F.). Correspondingly, the gasoline level and temperature must be measured to within 0.1" and 2.0.degree. F., respectively. In addition, a system must be able to detect product losses as small as 0.1 gallons per hour. In a typical UST this last requirement means that the system must be able to detect differential changes in fuel level to within 0.0005 inches and differential changes in temperature within the UST to within 0.005.degree. F.
These regulations impose extraordinary performance requirements on the accuracy, resolution and repeatability of the fuel level measurement. In order to obtain these performance requirements, more recent ultrasonic ranging systems have modified the reference reflectors and signal discrimination electronics in the systems in an effort to better discriminate between the primary echo signals and the secondary echo signals created within the systems, as well as to better detect the exact time in which a primary echo signal is received. Examples of these systems are seen in U.S. Pat. Nos. 4,748,846, 4,805,453 and 4,984,449.
However, even these most current prior art systems do not provide the performance nor the function capabilities necessary to determine the fuel volume in a tank, the leak rate from the tank, or maintain the inventory record for the tank being monitored as is required by the EPA. There still exists a need for an ultrasonic ranging system which can more accurately take into consideration the operating environment within a UST when making such measurements.